The present invention relates to an optical recording medium comprising a recording layer formed from a phase-change material changeable between a crystalline state and an amorphous state, which responds to a focused spot of a light by switching between crystalline and amorphous phases to thereby record an information signal, and to a method of manufacturing the optical recording medium.
Conventional optical recording media in which a variety of information signals can be written and/or read out with respect to a portion of the recording medium under the focused spot of a light such as a laser beam include: optical discs, in which information signal is previously written with embossed pits; phase-change optical discs, in which information signal is written to a recording layer thereof utilizing the switching between crystalline and amorphous states of the recording layer; and magneto-optical discs, in which information signal is written to a recording layer thereof utilizing the magneto-optic effect of the recording layer, etc. Each of these optical discs comprises a transparent substrate made of glass or a plastic, such as polycarbonate, having functional layers, such as a recording layer, a reflective layer, etc. formed on the substrate.
Among the types of optical recording media, the magneto-optical disc and phase-change optical disc are of a rewritable or erasable type. Especially, the phase-change optical disc is advantageous in that it is easily rewritable. Disc drives for the phase-change optical disks may be made very compact, since read and write functions with respect to the phase-change optical discs can be achieved without the need for apparatus for generating an external magnetic field. Thus, the phase-change optical disc has been attracting much attention in the field of industry.
In such a phase-change optical disc, an information signal is written by focusing a laser beam having a high level of power (will be referred to as "recording power" hereinafter) onto a recording layer of the optical disc to heat the recording layer to a temperature higher than its melting point, and then cooling it quickly enough to freeze the portion of the recording layer under the focused spot of the laser beam in an amorphous state to form a recording mark for the information signal.
To erase the information signal recorded on the recording layer of the phase-change optical disc, a laser beam of a power level below that of the laser beam used to record the information signal, that is, a lower-power laser beam than the recording level, is focused at least onto the recording mark to heat the portion under the focused laser beam to a temperature higher than the crystallization point and lower than the melting point, and then the heated portion is cooled to recrystallize the portion, namely, change the recording layer portion from the amorphous state back to the crystalline state, and thus erase the amorphous recording mark.
Further, in the phase-change optical disc, since even a same portion of the recording layer under the focused spot of laser beam differs in reflectivity depending upon whether it is in a crystalline or amorphous phase, a difference in reflectivity between the crystalline and amorphous states can be detected, by focusing a laser beam having a lowest level of power onto the recording layer, to read out the recorded information signal.
The recording layer of such a phase-change optical disc is formed from a phase-change recording material such as a calcogenide containing a GeSbTe alloy (will be referred to as "Ge-calcogenide" hereinafter), a calcogenide containing an AginSbTe alloy (will be referred to as "Ag-calcogenide" hereinafter), or the like. It is well known that a phase-change optical disc having a recording layer made of a Ge-calcogenide is superior in durability against repeated recording to a one having a recording layer formed from a Ag-calcogenide.
As mentioned above, the phase-change optical disc having a recording layer made of a Ge-calcogenide is known to be superior in durability against repeated recording to a one having a recording layer made of an Ag-calcogenide. It is also known however that the characteristic of a signal read out after being repeatedly recorded a few times to tens of times is regionally deteriorated and the characteristic of a signal read out after being repeatedly recorded more than tens of thousands of times is deteriorated drastically.
Thus, this conventional phase-change optical disc is not able to assure a stability of a signal read out after being repeatedly recorded a small number of times such as a few times to tens of times and also not able to assure a quality of a signal read out after being repeatedly recorded a large number of times, such as more than tens of thousand of times. In effect, the conventional phase-change optical disc is not yet sufficiently reliable with respect to the write/read characteristics thereof under conditions of intended re-use.